In recent years, the miniaturization and weight reduction of electronic equipment and communication equipment have been rapidly progressing, and hence, there is also an increasing demand for miniaturization and weight reduction of electricity storage devices such as secondary cells used as drive power sources for such equipment. With a view to meeting such a demand, secondary cells having high energy density and high voltage, such as those represented by lithium ion secondary cells, have been developed and come into wide use as substitutes for conventional alkaline storage batteries, and further, electricity storage devices designed in attempts to achieve still higher capacitance and their members have also been proposed (Patent Documents 1 and 2).
On the other hand, larger electricity storage devices are desired for application in electric vehicles, hybrid vehicles and the like, but involve many problems for practical applications. In the case of lithium ion cells, for example, there is a problem in securing input/output characteristics that can provide them with safety to meet to an increase in an electrolyte as a combustible substance and can furnish them for practical applications. In addition to improvements in safety, increases in capacitance and output, therefore, also lie as problems of electricity storage devices to be solved from now on.
To realize providing an electricity storage device with higher capacitance and higher output, it is effective to lower the internal resistance. For this purpose, it is important to control the charge transfer phenomenon at each of interfaces between respective layers such as an electrode layer, collector and electrolyte layer (Non-patent Documents 1 and 2), and a variety of proposals have been made.
At an interface of a graphite negative electrode in a lithium ion cell, for example, a surface resistance film formed by decomposition of an electrolyte, decomposition of a lithium-containing compound as a supporting salt or the like and called “an SEI film” gives serious effects on the maintenance of the performance of the cell. The addition of catechol carbonate or, its derivative to the electrolyte, however, results in an SEI film of smaller thickness, and hence, of lower film resistance (Non-patent Document 2).
In a lithium ion cell, an aluminum positive electrode collector forms a barrier film on its surface and is passivated in a fluorine-containing electrolyte, and this passive film affects the cycle characteristics. It has, however, been succeeded in imparting conductivity to such a passive film by subjecting an aluminum collector to heat treatment and applying a dispersion of ultrafine particulate carbon onto the heat-treated aluminum collector (Non-patent Document 3).
For improvements in safety, on the other hand, all-solid-state lithium ion cells making use of nonflammable electrolytes are drawing interest as electricity storage devices equipped with safety. These all-solid-state lithium ion cells are, however, accompanied by a drawback in that their output performance is still not sufficient. A report has been made recently that various problems associated with an interface between an electrode layer and an electrolyte layer, said interface taking a part in the rate-controlling step of a power output, was studied to solve the above-mentioned drawback, and based on the results of the study, an attempt was then made to interpose an oxide solid electrolyte as a buffer layer between the electrode layer and the electrolyte layer, leading to significant improvements in output performance (Non-patent Document 4).
In electricity storage devices containing solid electrolytes, fluorinated polymers the electrical conductivities of which are close to those of liquid electrolytes are used. These fluorinated polymers, however, involve a problem that they do not sufficiently come into close contact with collector metals. A proposal is thus disclosed, in which a collector is coated with an acid-modified polyolefin to solve the above-mentioned problem, and in addition, also to maintain excellent cycling characteristics (Patent Document 3).
In these proposals, however, the problems to be solved are limited to those arising under specific conditions. Many of them are, therefore, limited and individual solutions. As a reason for this situation, there are presumably still many unknown matters in the phenomena of charge transfer and ion transfer at the interfaces between the respective layers in an electricity storage device (Non-patent Document 3). Taking practical utility as a first essential point under these circumstances, the present inventors came to consider that it would be necessary to broaden the point of view, to take the phenomenon between each two constituent units in an electricity storage device as a single system including their interface, to study the reasonability of this system, and to attempt solutions to problems.
As a primary “system including an interface” in an electricity storage device, there is each electrode plate. Each electrode plate gives considerable effects on the performance of the electricity storage device, and is an electrode member with unit members such as an electrode layer and collector integrated therein. Concerning such an electrode plate, proposals have been made to permit its production in the form of a thinner film with larger area such that it can be provided with an extended charge-discharge cycle life and an increased energy density. As to lithium ion cells, for example, Patent Document 4, Patent Document 5, etc. disclose positive electrode plates each of which is obtained by dispersing or dissolving an electrically conductive material (hereinafter referred to as a “conductive material”) and binder along with powder of a positive-electrode active material such as a metal oxide, sulfide or halogenide in an appropriate solvent to prepare a paste-form coating formulation, providing as a substrate a collector formed of a foil of a metal such as aluminum, and applying the coating formulation onto a surface of the substrate to form a coating film layer.
A capacitor, which makes use of an electric double-layer formed at an interface between a polarizable electrode plate and an electrolyte, is used as a memory backup power supply, and its use in fields that require large outputs like a power source for an electric car is also attracting interests. For large outputs, this capacitor is hence required to have both a high capacitance and a low internal resistance. Like a negative electrode plate for the above-described cell, the electrode plate for the capacitor is produced by applying onto a collector a coating formulation, which is generally a mixture of a binder and conductive material, and then drying the coating formulation.
As the binder for use in the coating formulation for the electrode plates in the above-described electricity storage device such as the lithium ion cell or capacitor, a fluorinated resin such as polyfluorinated vinylidene or a silicone-acrylic copolymer is used, for example. A negative electrode plate (cell) or polarizable electrode plate (capacitor) is obtained by adding a solution of a binder in a suitable solvent to an active material such as a carbonaceous material to prepare a paste-form coating formulation and then applying the coating formulation onto a collector. In the above-described coated electrode plate, the binder employed to prepare the coating formulation is required to be electrochemically stable to a nonaqueous electrolyte and to be free from dissolution into the electrolyte of the cell or capacitor, to remain free from substantial swelling by the electrolyte, and further to be soluble in a certain solvent to permit the coating.
On the other hand, it is practiced to form a protective film on a surface of a metal material such as aluminum, as a base metal material of a collector, by coating a solution of one of various resins. The resulting film is excellent in the adhesiveness to the metal surface, but is accompanied by a problem in that its durability to an organic solvent is insufficient.
In the electrode plate for the cell or capacitor, said electrode plate having been obtained by applying the above-described coating formulation onto the surface of an aluminum foil, copper foil or the like as the collector, the coating film layer formed by the coating and drying is accompanied by problems in that its adhesiveness to the collector and its flexibility are insufficient, the contact resistance between itself and the collector is high, and peeling, flaking, cracking and/or the like of the coating film layer takes place during assembly steps of the cell or capacitor or upon charging and discharging the same.
As described above, the conventional cell or capacitor is accompanied by the problems of the poor adhesion between the electrode layer and the collector (substrate) and the high internal resistance at the interface between the electrode layer and the substrate. A variety of coating formulations have been proposed to solve these problems. Coating film layers formed from these coating formulations lessen the adhesiveness problem, but make still higher the resistances between the electrode layers and the collectors. Therefore, none of these coating formulations have led to a solution to the problems yet. In recent years, there is also an increasing demand for the manufacture of the above-described electricity storage devices such as lithium ion cells and electric double-layer capacitors and their related products with due consideration being paid to the environment. There is hence a demand for a coating formulation and electricity storage device each of which uses components, materials and a preparation or manufacturing method that do not add much load on the environment load.